1. Field of the Invention
This invention concerns an ignition distributor of a type for making electrical connection through electrical sparkings and, more particularly, it relates to an ignition distributor having a noise preventing function of suppressing the generation of radiowave noises caused by spark discharges generated between a rotary electrode and stationary electrodes disposed on the side of the rotating circumference.
2. Brief Description of the Prior Art
Radiowave noises caused by spark discharges generated in an ignition system of an internal combustion engine mounted in automobiles or the likes give interferences to communication equipment such as television or radio receivers. The causes for generating the radiowave noises from the ignition system of the internal combustion engine mainly include the following three types, that is, (1) spark discharges between electrodes of an ignition plug, (2) spark discharges between the rotary electrode and the stationary electrodes of a distributor and (3) spark discharges due to the switching operation of the breaker points in the distributor.
Among the causes described above, while the following countermeasures (I)-(V) have been proposed as the means for preventing the radiowave noises caused by (2) above, they have drawbacks respectively and cannot attain a sufficient effect.
(I) Method of using a rotary electrode incorporated with resistive material,
This method uses a rotary electrode embedded with a resistor. However, since a distributed capacitance is present in parallel with the resistor, its noise suppressing effect is decreased at a high frequency wave region of about more than 300 MHz and also has a drawback that there is a large loss in the ignition energy due to the resistor (about several kiloohms). Furthermore, its noise suppressing effect is as low as about 5-6 dB even in the frequency region of lower than 200 MHz for which the noise suppressing effect can be expected.
(II) Method of using a rotary electrode applied with flame coating
This method uses a rotary electrode applied at the surface thereof with a high resistive material layer. However, this method has drawbacks, for example, in that (i) since a highly resistive material layer is formed to the surface of the electrode, the loss in the ignition energy is remarkable and (ii) the noise suppressing effect is as low as 5-6 dB in the frequency region lower than 200 MHz.
(III) Method of enlarging the discharging gap
In this method, the discharging gap between the rotary electrode and the stationary electrode is enlarged to about 1.5-6.4 mm. Although this method is advantageous in that it provides a noise suppressing effect as high as 15-20 dB, the loss in the ignition energy is extremely high because of the extremely large discharging gap, and gases corrosive to metals, such as nitrogen oxide (NO.sub.x), are generated to corrode the rotary electrode since the discharging voltage between the electrodes becomes higher.
(IV) Method of using boride, silicide, carbide and electroconductive ceramics (specific resistivity from 10.sup.-6 -10.sup.-2 ohm cm) for the electrode
While these substances show a low ignition energy loss because the resistance of the electrode is relatively low, the noise suppressing effect in the frequency region of lower than 300 MHz is as low as about 5-10 dB, as well as the electrode is liable to be consumed due to the local discharge since these substances are poor in the heat conduction.
(V) Method of using electroconductive ferrite for the electrode
This method can provide a satisfactory noise preventing effect as high as 10-15 dB. However, since the substance has a relatively small specific resistivity at low frequency, large current flows to cause heat generation by the induction discharge through the discharging gap. Further, since the substance has a poor heat conductivity, the electrode is locally consumed upon heat generation caused by discharge. On the other hand, in the case of using a ferrite having high specific resistivity, although the noise preventing effect and the durability are satisfactory, the loss in the ignition energy is higher.